How to be a great salesperson?

I have an interview at a clothing store tomorrow whose pay is competively commission-based. This will be my 1st job if I get it. I have done many mock interviews at school, so the interview is not the problem. I am very worried about being a good seller. How do I persist in getting someone to buy something? What are the signs to back off? What are some phrases I should know & use constantly? (Ex. Hi, is there anything I can help you find today?) I really want to be good at sales, especially since my pay will be mostly based off how much I sell. Any advice would be greatly appreciated!